Physical vapor deposition in a vacuum environment is the principal means of depositing thin films of material, such as the thin organic material films used in small molecule organic light-emitting diode (OLED) devices. Physical vapor deposition methods are well known, for example in U.S. Pat. No. 2,447,789 to Barr and in and in EP 0 982 411 Tanabe et al. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate-dependent vaporization temperature for extended periods. Exposure of sensitive organic materials to high temperatures can cause changes in the structure of the molecules and unwanted changes in material properties.
Sources is accomplished by placing a quantity of vaporizable material in a source and heating it to a very well controlled and constant temperature. Because the temperature stability directly influences the deposition rate stability, it is very important to maintain a constant source temperature. The relationship between vapor pressure, and hence deposition rate, versus material temperature for two organic materials is shown in the graph of FIG. 1. From FIG. 1, it is apparent that, over a range of values, a small perturbation in source temperature can cause a sizable perturbation in vapor pressure. This amplification is particularly pronounced at higher temperatures.
In light of this temperature-to-pressure relationship, conventional vapor deposition sources have a relatively large thermal mass that is useful in minimizing temperature fluctuations. However, as a result, it can require many hours to achieve an equilibrium temperature and a stable vapor deposition rate when using this conventional approach. Due to the relative thermal sensitivity of organic materials, the conventional approach has been to load only small quantities of organic materials into a source at a time and to apply as little heat as possible. Drawbacks of this conventional process include the loss of some portion of the material before it has reached the temperature exposure threshold, a very low vaporization rate due to the limitation on heater temperature, and limited operation time of the source due to the small quantity of material present in the source. Using this prior technique, it has been necessary, when recharging a source, to vent the deposition chamber, disassemble and clean the vapor source, refill the source, reestablish vacuum in the deposition chamber and degas the just-introduced organic material over a period of several hours before resuming operation. The low deposition rate and the frequent and time consuming process associated with recharging a source has placed substantial limitations on the throughput of OLED manufacturing facilities.
One alternative to conventional vapor deposition methods is the use of a flash vaporization system utilizing a feeding mechanism to deliver vaporizable material to a heating element where it is rapidly vaporized. Referring to the perspective diagram of FIG. 3, a feeding apparatus 20 provides a continuing supply of a vaporizable material 22 into a deposition chamber 24 for forming a layer on a surface 26. A heating element 30 provides the needed vaporization energy to the vaporizable material 22. Feeding apparatus 20 effectively provides a temperature controlled region that maintains the vaporizable material 22 below its vaporization temperature in order to prevent degradation or breakdown of the vaporizable material 22 before it reaches heating element 30. Feeding apparatus may use an auger or other mechanism for providing a constant supply of vaporizable material 22. A severe limitation of prior art flash vaporization systems, where the evaporable material is not in liquid form, is the inability to maintain a steady vaporization rate because even small variations in the delivery rate disturb the vaporization rate. Because of this rate stability problem, the conventional approach teaches away from flash vaporization systems, even where vaporizable material is continuously metered to a heating element. As just one example of the bias against flash vaporization, Loan in U.S. Pat. No. 6,296,711 specifically teaches away from the use of flash vaporization system, preferring instead to distribute the dispensed evaporable material over a cone shaped heating element having an ever-increasing surface area.
Recently, flash vaporization PVD sources have been developed that employ a very low thermal mass heating element for vaporizing material. Advantageously, this allows a material that is being metered to be maintained at a temperature that is well below the material's effective vaporization temperature. However, there is a significant difficulty with flash vaporization techniques. When using such a system, the vaporization rate is directly related to the material feed rate for a constant heating element temperature. Perturbations in the material feed rate to the flash vaporization heating element, with the heating element maintained at a constant temperature, are seen directly as perturbations in the resulting vapor deposition rate. Higher frequency perturbations may be attenuated where a manifold is employed to distribute the vapor over a length or area, as long as the period of the perturbation frequency is higher than the residence time of the vapor in the manifold. Lower frequency perturbations in the feed rate, however, can be more serious, leading to thickness non-uniformity of the deposited films in scanning type sources.
Those skilled in the materials metering arts can well appreciate the difficulty of obtaining a constant feed rate with minute quantities of materials. Many prior art deposition sources used in the manufacture of OLED devices deposit organic thin films at a rate on the order of 100 μg/s or less. In deposition systems where the OLED substrate is scanned past the source or vice versa, it is necessary to maintain a constant deposition rate in order to achieve film thickness uniformity of the deposited film. Typically, film thickness uniformity for OLED fabrication must be better than +/−5%. Using a feeding mechanism in cooperation with a flash vaporization system at these deposition rates would require feeding uniformity of +/−5 μg/s. This precision level of uniformity would be extremely difficult to achieve using, any of today's known metering technologies for any materials not in liquid form.
One approach for minimizing the effect of low frequency perturbations in the feed rate is to employ a conventional closed loop feedback scheme. A manifold pressure sensor or deposition rate sensor can be used as a feedback element to the closed loop control system to adjust the feed motor speed and the heating element temperature to attain a constant deposition rate. Feed forward control schemes can be used advantageously in conjunction with feedback schemes where there is a known and predictable periodicity to the material feed rate. In this case, a motor speed profile can be preprogrammed to compensate an assignable variation in metering device performance. Adjusting the material feed rate alone however, provides only limited deposition rate control because there may be a time lag of several seconds between a control signal to the feed motor and the resulting change in deposition rate. In addition, material feed rate is not a bi-directional control variable. Material already metered to the heating, element cannot typically be retrieved if the deposition rate rises above the desired control limit.
The other factor that could be controlled in a closed loop system is the heating element temperature. In contrast to the feed rate, change to the heating element temperature produces an almost instantaneous change in deposition rate and is bi-directional. The influence of temperature change, however, can only be applied over a short time interval. If the heater temperature is driven too high or too low for an extended period of time there can be a material-starved or material-overload condition at the heating element.
With these considerations, it is evident that closed loop control would require controlling both the material feed rate to the heating element as well as the heating element temperature in order to produce a constant deposition rate. Such a closed loop, multi-variable control method is relatively complex as it requires a relationship to be maintained between the material feed rate and the heating element temperature, and it requires tuning to optimize numerous gain settings and requires sensing of material feed, heating element temperature, and vapor deposition rate.
For OLED fabrication, pharmaceutical manufacture, and a number of other applications, there is a need for vapor deposition apparatus and methods that provide continuous operation and highly uniform results without the need for complex and expensive sensing and control components.